Blue LEDs Fail Color Differentiation Test

Photonics.comApr 2014
STATE COLLEGE, Pa., and FREMONT, Calif., April 23, 2014 — Not all shades of white are identical, although traditional and some LED lighting can make this difficult to prove.

A study by researchers from Pennsylvania State University and Soraa Inc. focused on various combinations of light sources and white objects to determine how those light sources affected people’s perceptions of white.

Participants in the study completed sorting, forced choice and selection tests using five light sources: a filtered halogen lamp, a common blue-pumped LED and three violet-pumped LEDs with differing levels of violet emissions.

A set of white cards as seen under a violet-pumped LED (top), and a blue-pumped LED (below). Images courtesy of Penn State.
The sorting test, in which participants were asked to arrange six calibrated cards of varying shades in order of whiteness, showed that the halogen lamp allowed for the most accuracy, as did the two LEDs with the highest violet emissions, 7 percent and 11 percent.

“People simply couldn't tell the difference between the cards under the blue-pumped LED, which is notable because blue-pumped LEDs are by far the most common type for general lighting,” said Kevin W. Houser, a professor of architectural engineering at Penn State.

The higher-violet-emission lights offered the most accuracy for participants in the forced choice test, in which they needed to determine the whiter of two cards.

In the selection test, the researchers asked participants to look at a reference card in one area and then determine if other cards in another area were as white or whiter. As with the other two tests, the common blue-pumped LEDs hindered accurate visualization.

“Retailers have long been concerned with the color-rendering qualities of their lighting, but less aware how light sources render white,” Houser said.

He and fellow researchers agree that “engineering of an LED source's spectrum is necessary for an accurate rendering of whiteness,” especially as such lighting continues to replace long-used incandescent or fluorescent bulbs.